Image Texture Node Output
Alpha datas are useful when you want to give transparency to an area of your material.

In the case of the Image Texture Alpha output, let say you have a PNG with a transparent background (0% alpha). Plug the Image Texture's Alpha output into the Alpha socket of the Principled (or into the Factor of a Mix Shader between a Diffuse and a Transparent shader), the alpha of your image will be recognized as the area that needs to be transparent.